Trace metal geochemistry of metalliferous black shales in the Mamfe basin (South West Cameroon): Implication for heavy metal assessment

Abstract

This study is focused on assessing the contamination levels of heavy metal elements (Cu, Cd, Cr, Ni, and Zn) in black metalliferous shales of the Mamfe basin in South Western Cameroon and identifying the possible pollution sources. Forty (40) black shale samples collected from seven outcrops from different sites in the Mamfe basin were subjected to geochemical analysis by inductively coupled plasma-mass spectrometry (ICP-MS). The results obtained from these analyses were used to calculate the contamination factor (CF), geo-accumulation index (Igeo), and Pollution Load Index (PLI). Person's correlation coefficient was applied in this work to assess the metals origin. The values of heavy metals in the shales were compared with experimental values of TEL (Threshold Effects Level), PEL (Probable Effects Level), ERL (Effects Range Low), and ERM (Effects Range Medium) to assess the long-term adverse biota effect if the shales weathers and release the hosted heavy metals. The average heavy metal concentrations in the studied shales were 35 mg/kg, 89 mg/kg, 0.24 mg/kg, 82 mg/kg and 37.3 mg/kg for Cu, Zn, Cd, Cr and Ni respectively with some above the threshold values of United State Environment Protection Authority (USEPA), average shale (ASV), toxicity reference values (TRV) and Indian River system (IRS). The shales are poorly to moderately enriched in heavy metals which may cause a long-term effect on the surrounding soils and waters after degradation. The heavy metal values of the studied shales compared with the experimental results of TEL, PEL, ERL, and ERM indicate that a little adverse biota effect of Ni, Cu and Cr to the ecosystem after degradation of the studied shales. The geo-accumulation index ranged between 0 and 0.06 indicating that the black Mamfe shales are uncontaminated to moderately contaminated and may have adverse effects on the freshwater ecology and surrounding soils. The pollution load index (between 0.0 to 0.05) and contamination factor (0.18-1.84) indicate that the shales have a low level of contamination and their long-term degradation and weathering will have a limited effect on the soils and water ecosystems. The positive correlation between Cu, and Zn, suggest the same source of contamination input. The positive correlation between Zn, Cu, Cr, and Ni indicates a natural origin of these elements in the shales. A rapid weathering/degradation of large quantity of theses shales may increase the concentration of heavy metals in the surrounding rivers and soils in the near future.